Electrolytic polymerization of phenol



Unite States This invention relates to the electrolytic polymerization and deposition of aromatic phenols which retain their aromatic structure when partially oxidized. These aromatic phenols in accordance with the invention are subjected to electrolytic actionin the molten state or in solution in non-aqueous solvents. The invention also relates to new and improved coating compositions for metal surfaces, methods of preparing such coatings, and methods for applying them to metals. The invention relates particularly to the elec-trodeposition of strongly adherent, insoluble films, especially passivating films, of oxidation products having an aromatic structure on .metal surfaces, the metal serving as the anode in the electrolytic bath, and to the application of such films in the coating of metal surfaces. The invention also relates to methods of plasticizing such films and to the resulting plasticized films. 1

Adherent, insoluble coatings providing corrosion protection for metal surfaces have long been sought, par ticularlyfor iron surfaces, and the prior art has-made use of various types of synthetic resins for that purpose. The majority of such prior art resins, however, are subject to attack by many chemicals, such as strong acids, and are subject to decomposition at temperatures in the general vicinity of 500 F. or less.

We have found that the electrolytic deposition products of aromatic phenols which retain their aromatic structure when partially oxidized may be prepared in the form of films on certain metal surfaces, and that such films exhibit in high degree the desirable properties of inertness toward strong chemical reactants and solvents, even at high temperatures, and resistance to decomposition at temperatures as high as 680 F.

The films are thus very useful in lining containers or coating articles which must hold or be exposed to such chemicals at elevated temperatures. More particularly, films may be formed on metal surfaces subject to corrosion by chemical agencies, to provide strongly adherent, physically and chemically inert and insoluble protective coatings on the interior of metallic cans for food or chemicals, tanks, kettles and other process equipment for conducting chemical and related processes and for storing the products thereof. The said oxidation products are deposited directly on certain metal surfaces when the metal is lthe anode in the electrolytic cell, and consequently cah be produced in a very simple manner. The resulting coatings are resistant to water, atmospheric influences, and corrosion.

The new electrolytic deposition products are prepared by the electrolysis of aromatic phenols which retain their aromatic structure when partially oxidized, ormixtures thereof, serving as a substantially non-aqueous electro.- lytic bath either in the molten'state or in solution in a substantially non-aqueous solvent or mixture of such solvents. By substantially nonaqueous is. meant a water content not exceeding about one percent.

The aromatic phenols which retain the aromatic ben- 'zenoid structure in the oxidized condition in accordance with anodic oxidation by the present invention exclude those dihydroxy substituted aromatic compounds. which produce, on oxidation, the ortho or para quinoid struc+ ture.

atnt For example, hydroquinone and catechol which produce quinones on oxidation are incapable of forming the valuable films of the invention. However, the substitution of an alkyl group for one of the phenolic hydrogen atoms of either ortho or para dihydroxy benzene, by preventing the production of a quinoid structure upon oxidation, provides a suitable phenol starting material for anociic deposition by the invention.

The aromatic phenols which retain their aromatic structure in the oxidized condition are conventionally-the mono-nuclear and bi-nuclear aromatic compounds in the benzene and naphthalene series as well as the hydrocarbon and halo-gen substitution products of these.

Thus, it is preferred to employ phenols of the benzene and naphthalene series which are not otherwise substituted than with hydrocarbon and halogen and which re tain this aromatic structure in the partially oxidized condition.

Typical phenols which are preferred for film formation in accordance with the-invention to form tightly adherent, inert, insoluble and heat-resistant solid cross-linked polymeric coatings in accordance with the invention include phenol, o-chlorophenol, o-hydroxy diphenyl, 3,5-xylenol, m-cresol, m-dihydroxy benzene, 3-methyl, S-ethyl phenol, p-p' dihydroxy diphenyl, p-tertiary amyl phenol, 2,5 xylenol, bis-phenol A, p-phenyl phenol, alpha-naphthol, and 1-7 dihydroxy naphthalene. These preferred phenols contain a phenol hydroxyl group which exerts its directive (orthoor para-directive) influence towards at least two unsubstituted positions in the aromatic ring of said phenols.

Hydrocarbon substituted phenols which are substituted in the ortho position relative to the phenolic hydroxyl group are generally suitable when the hydrocarbon group is a normal or branched chain alkyl group or an aromatic group such as phenyl or naphthyl. However, if the carbon atom of the alkyl group which is linked to the am} matic ring in the ortho position to the phenolic group a tertiary carbon atom, the so substituted alkyl group appears to interfere with attainment of adherent films on the metal anode. I

The phenols in accordance with the invention contain at least two unsubstituted positions in the aromatic ring, said unsubstituted positions being preferably either ortho or para to the phenolic hydroxyl group of said phenol.

Particularly desirable films possessing enhanced inertness to chemical attack and resistance to high tempera ture coupled with tenacious adherence to the underlying metal surface are produced in accordance with the in vention by the anodic oxidation of phenols having at least one ortho position of the aromatic ring free from substitution and either a halogen substituent on the ortho posi tion or a tertiary carbon atom in the para position. This tertiary carbon atom may be supplied by an alkyl sub stituent as in para tertiary alkyl substituted phenols such as para tertiary butyl phenol, or by a phenyl substituent as for example para phenyl phenol.

These inert films are of particular value as protective linings for prefabricated metal products such as tanks and pipes used in chemical industry for storage of corrosive liquids, or for conducting chemical reactions :and for conveying reactants, reaction mixtures and products The inert films of the invention possess unusual appli; cability in situations where the protective lining is to be applied in places which arenot readily accessible orwhefg elevated temperatures must be resisted. Thus, assembled chemical process equipment or elements thereof of com-1 plex configuration may be provided with. uniquelyincltt and high temperature resistant protective films -lby ms mersing the part or parts to be protected in the phenol bath and making these parts the anode of an electrical circuit flowing through the said bath. The protection of corrosion-resistant pipes can be achieved with particular case by immersing the pipe in the phenol bath, making .the pipe the anode of an electrical circuit flowing through the bath and inserting the cathode coaxially through the pipe.

Without adopting any particular theory of action, one possible explanation of film formation is that the electrolysis of molten phenolic compounds, or of non-aqueous solutions of such compounds yields on the anode passive films which are formed by the discharge of phenolate ions.

The films which are produced in accordance with the invention can be considered as cross-linked polymeric compounds comprising a plurality of phenylene groups.

In some instances and under certain circumstances, the solid products formed during electrolysis tend to separate from the metal anode and the invention contemplates the incorporation of plasticizing materials to enhance adhesion and to improve the flexibility of the deposited films.

In accordance with the present invention, it has been found that this result may be obtained if there is added to the phenol, preferably in the electrolytic bath, a plasticizing substance which contains a hydroxy group. Such compounds include both substituted phenols and alcohols, as for example, ortho-cresol, meta-cresol, cyclohexanol, and furfuryl, benzyl, n-hexyl and allyl alcohols. It has also been found that certain long chain esters may be used as plasticizers. Examples of such esters are esters of lower monoor di-hydric aliphatic alcohols with a 9-carbon atom straight chain or aliphatic fatty acid.

A cell found to be suitable for conducting the electrolytic polymerization and deposition is one in which graphite serves as the cathode, while the anode is composed of a suitable metal to be coated, in the form of sheets, plates, or other forms. Examples of metals which may be coated with passivating films by the process of the invention include iron and its alloys (including cast iron, wrought iron, and various steels), copper and zinc. Suitable zinc surfaces include the iron-zinc alloy formed by sherardizing zinc on a ferrous base. The film of the invention does not deposit on lead or tin.

The initial voltage applied to the cell may range from about 30 to 110 volts or more, preferably 50 volts. The initial current density may vary from about 5 amperes per square foot to about 50 amperes per square foot, preferably 30 amperes per square foot. perature is maintained at from 113 F. to 167 F., preferably about 140 F. Current efiiciency ranges from 50 to 70 faradays per mole of phenol deposited. The time required for coating deposition may vary from 10 minutes to 60 minutes, usually about 40 minutes.

In the course of the electrolytic oxidation the progress of the coating operation may be observed by (a) color changes, and (b) increased passivity of the metal.

It has been found that, in the case of metals which accept a coating, there at first appears a trace of iridescence, followed by stronger manifestations of iridescence ranging in color from yellow-green to blue-green. The occurrence of such iridescence will depend, however, on the operating conditions chosen. The color of the coating itself progresses from a golden or tan film, on through yellowor greenish-blue, to blue and finally dark blue. It will be apparent that decorative effects can be thus ob tained.

Upon passing an electric current through the phenol electrolyte, the product deposited upon the anode undergoes, in many instances, a change in color from the time of initial film formation continuing to the passivation condition of the deposited film. During this period of deposition, the film increases in thickness. Accordingly, the color of the deposited film can serve as a measure of film thickness deposited anodically and this can serve several valuable functions. For example, film color can beused to insure: (1) the presence of a minimum thickness; (2)

The cell tem- A the absence of an unnecessary or undesirable thickness; and (3) the provision of a uniform thickness.

The degree of passivity of the deposited film on the metal anode during the electrolytic oxidation may vary, time and current factors remaining constant, on the basis of the phenol electrolyte which is selected. Also influencing the degree of passivity of the deposited films is the character and amount of addition to the electrolyte, such additions including solvents, alkalies, and alkaline reacting alkali metal salts such as alkali metal salts of weak organic or inorganic acids.

Films obtained from phenol and o-chlorophenol, for example, are insoluble in the following solvents: water, ether, acetone, 10% aqueous NaOH, concentrated H 50 dilute HCl, dilute HNO ethyl cellosolve, ethyl acetate, acetic acid, molten phenol, and liquid formamide. The o-chlorophenol film is also insoluble in morpholine and dioxane.

In order for satisfactory films to be formed from any phenol, water cannot be present in amounts in excess of about 1% by volume. Water in excess of this limit tends to destroy the iridescent character of the film, and to cause poor adherence to the anode.

In order to increase the conductivity necessary for film formation from phenols in accordance with the invention, small amounts of alkali metal hydroxides, such as sodium, potassium, and lithium hydroxides, ranging from 2 to 5 g. per mole of the phenol may be added to the electrolyte. Excess alkali tends to result in perforated films. Salts of alkali metals may also be added to increase the conductivity of the cell, particularly those in which the volume preempted by the anion in solution is large in comparison with the volume preempted by the alkali metal cation and which have a decomposition potential which exceeds that of water. The alkali metal salt is generally used in amounts of about 2 grams of salt to 1.5 moles of the phenol. Examples of salts which may be used successfully are lithium acetate, sodium carbonate, sodium acetate, sodium phosphate, and sodium borate. If necessary in order to secure solubility of the salt, methyl alcohol may be added.

As previously indicated, the phenol may be anodically deposited from the molten state or from solution in sub stantially non-aqueous solvents. Examples of such solvents include alcoholic solvents such as methanol, ethanol, ethylene glycol, 50/50 mixture of methanol and glycol, glycerol, as well as nitrobenzene, N,N-dimethylformamide, acetone, and urea. Methanol is preferred.

Where desirable, small amounts of alkali metal hydroxide may be added to the solution of the phenols in the non-aqueous solvents in order to provide increased conductivity.

The invention is illustrated by the following examples:

EXAMPLE 1 Efiect of electrolysis time on final current and film color Experiment Electroly- Initial Final Cur- Designation SIS Time, Current, rent, Film Color min amps. amps.

0.6 5.0 4.0 tan, 1! ht blue. 1.8 5.0 3.0 light b lue. 4. 0 5. 2 2. 2 yellow, blue. 9. 5 5.9 1.2 green, blue. 17. 8 l5. 7 0. 6 blue. 41. 5 6. 1 0. 1 dark blue.

The above table shows that the flow of current through the cell diminishes with time, and after about 10 minutes drops from an initial value of from to 6 amperes, to about 1 to 2 aniperes, indicating that the iron electrode has become passive. At the same time color changes take place on the iron electrode, depending on the length or time of the electrolytic v oxidation. The color progresses from a light tan or light blue to a dark blue when the electrolysis is complete.

EXAMPLE 2 Using the same general conditions as in Example 1, the electrolysis was conducted with copper and zinc anodes. The initial current density for the copper anode was 15 amperes per square foot, and the initial current density for the zinc anode was 50 amperes per square foot. Both electrodes became passive and films were deposited on the metal.

EXAMPLE 3 Using ochlorophenol as the molten electrolyte and graphite as the cathode, and with sheet iron as the anode, with a voltage of 80 volts, a current density of 7 amperes per square foot, the cell temperature being about 140 F., films similar to those obtained in Example 1 were obtained on the iron sheet.

The passivating films obtained in the foregoing examples are extremely hard, adherent, and inertto most common solvents and oxidizing agents. When subjected to molecular distillation and pyrolysis, such as by heating at around 680 F. for about six hours under high vacuum, no distillation products are observed and no pyrolysis product is found. The film material appears to be unchanged by this treatment. On being subjected to oxidation by suspending 200 mg. of the film in ml.. of glacial acetic acid to which was added 10 ml. of 30% hydrogen peroxide solution, and heating the mixture for two days, the starting material may be recovered unchanged.

The film may be separated from the metal plate by dissolving the metal with a strong aqueous solution of hydrochloric acid, the film coming ofi in small pieces.

Chemical analysis showed the composition of the phenol film to be about 74% C, 4% H, l-2% ash, and the remainder presumably oxygen. If the percentage impurity is assumed to be about the percentage ash, the film empirical formula becomes C H O, indicating that the material is a phenolic polymer.

The following examples illustrate the use of nonaqueous solutions of phenols as electrolytes in coating formation:

EXAMPLE 4 A solution of 2.17 moles of phenol in 0.43 mole of ethylene glycol, containing 2 g. of KOH was electrolyzed using a graphite cathode and a steel plate anode. The impressed voltage was 50 volts. The temperature of the cell was about 140 F. The current density was about amperes per square foot. The time of the electrolysis was about minutes, during which the current dropped to less than 1 ampere per square foot. A yellow-green iridescent coating was obtained.

EXAMPLE 5 Using conditions similar to those in Example 4, but with the electrolyte composed of 1.49 moles of phenol dissolved in 0.75 mole of acetone, with addition of 1 g. KOH, the passivity occurred and a smooth brown adherent coating was obtained.

EXAMPLE 6 Using conditions similar to those in Example 4, with an electrolyte comprising 1.5 moles of phenol and 0.30 mole of urea with addition of 5 g. of KOH, passivity occurred and an iridescent coating was formed.

6 EXAMPLE 7 Using conditions similar to those in Example 4, and with an electrolyte comprising 1.28 moles of phenol dissolved in 0.69 mole of absolute ethyl alcohol, with addi tion of 3 g. KOH, a blue-green iridescent coating resulted. Among the previously mentioned plasticizing agents, ortho-cresol and meta-cresol have been found to be particularly valuable, and when used in conjunction with phenol and caustic soda electrolyte in amounts ranging from 10 to 35% they markedly improve the flexibility of the phenylene oxide films. Paracresol produces adverse effects. A flexibility test run on a coated strip of iron demonstrates that additions of orthoand meta-cresol made it possible to bend the coated strip without causing the coating to flake. Dipping the metal in copper sulphate solution of 5% strength results in the deposition of no more than a small amount of copper.

The use of plasticizers of the type described istrated by the following examples:

EXAMPLE 8 EXAMPLE 9' Using the same conditions as in Example 8 but with an electrolyte of 73% phenol, 25% meta-cresol, and 2% sodium hydroxide, films were deposited which possessed;

illusbetter flexibility than the films deposited from a plain phenol electrolyte.

EXAMPLE 10 Using an electrolyte comprising 78% phenol, 20% benzyl alcohol and 2% sodium hydroxide and with all" other conditions being the same as in Example 8, films were deposited on the iron sheet. The flexibility of these films was superior to the same property of films deposited from an electrolyte containing only phenol.

Although the foregoing examples describe the we ferred utility of the cross-linked solid polymers as films adhered by anodic deposition to metal bases, the poly-' meric solid material may be used for other purposes where heat-resistant and inert products are desired. For example, the flaky inert solid produced by anodic deposition of phenol in the present of 2%-5% para cresol which fails to adhere to the metal base may be separated from the electrolytic bath in which it appears and utilized with good results as a component of a heat-resistant coat ing. Thus, by incorporating the separated solid polymer into a heat-resistant binding material such as a-silieone resin, useful coating materials can be produced. Suitable silicone resins are silicone-alkyd resin combinations in which a methyl, ethyl or phenyl silicone is co-condensed with an alkyd resin from phthalic anhydride and glycerin, the siloxane of the silicone having sufiicient free hydroxyl groups to chemically combine with the free hydroxyl groups of the alkyd resin.

While we have described present preferred embodiments of the invention and methods of practicing the same, it will be recognized that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

We claim:

1. An inert, insoluble and heat-resisting cross-linked polymeric solid produced by subjecting a phenol, which retains its aromatic structure when partially oxidized and which has at least two unsubstituted positions in the aromatic ring, said positions being selected from the group consisting of positions ortho and para to the phenolic,

7 hydroxyl group of said phenol, to a unidirectional electrical current, in a substantially non-aqueous electrolytic bath containing phenolate ions of said phenol, to .cause said phenolate ions to discharge and said phenol to polymerize, cross-link and be deposited at the anode.

2. A solid polymer as claimed in claim 1 in which said electrolytic bath includes an alkaline reacting material which is a derivative of alkali metal.

3. A solid polymer as claimed in claim 1 in which said phenol is mono-nuclear.

4. A solid polymer as claimed in claim 1 in which said phenol has a tertiary carbon atom linked to the aromatic ring para to the phenolic hydroxyl group of said phenol.

5. A solid polymer as claimed in claim 1 in which said polymer is in the form of a thin film.

6. An inert, insoluble and heat-resisting cross-linked polymeric solid film adhered to a metal base and produced by subjecting a mono-nuclear phenol, which retains its aromatic structure when partially oxidized and which has at least two unsubstituted positions in the aromatic ring, said positions being selected from the group consisting of positions ortho and para to the phenolic hydroxyl group of said phenol, to a unidirectional electrical current, in a substantially non-aqueous electrolytic bath containing phenolate ions of said phenol and said metal base as anode, to cause said phenolate ions to discharge and said phenol to polymerize, cross-link and be deposited as an adherent film upon said base.

7. A polymeric film on a surface of metal as claimed in claim 6 wherein the said metal is iron.

I 8. A polymeric film as claimed in claim 6 wherein the said metal is copper.

9. A polymeric film as claimed in claim 6 wherein the said metal is zinc.

10. A polymeric film as claimed in claim 6 in which said film is resistant to decomposition at temperatures of about 680 F.

11. A polymeric film as claimed in claim 6 in which said film includes a plasticizer incorporated therein, said plasticizer increasing the flexibility of said film.

12. A polymeric film as claimed in claim 11 in which said plasticizer is selected from the group consisting of hydrocarbon substituted phenols, alcohols and long chain fatty acid esters of lower aliphatic alcohols.

13. A polymeric film as claimed in claim 6 in which said phenol is a paraphenyl phenol.

I 14. A polymeric film as claimed in claim 6 in which said phenol is a para tertiary alkyl phenol.

15. A polymericfilm as claimed in claim 6 in which said phenol is ortho-chloro phenol.

16. A method of producing an adherent, inert, insoluble and heat-resisting cross-linked polymeric solid film upon a metal surface comprising immersing said metal in a substantially non-aqueous electrolytic bath containing an aromatic phenol having at least two unsubstituted positions in the aromatic ring, said positions being selected from the group consisting of positions ortho and para to the phenolic hydroxyl group of said phenol, said phenol retaining its aromatic structurewhen partially oxidized and said electrolytic bath comprising phenolate ions of said phenol, and passing a unidirectional electrical current through said bath andthrongh said metal anode to cause said phenolate ions to discharge and said phenol to polymerize, cross-link and be deposited as a polymeric film upon said metal.

17. The method recited in claim 16 in which said metal is selected from the group consisting of iron, copper and zinc.

18. The method recited in claim 16 in which said bath comprises said phenol in molten condition.

19. The method recited in claim 16 in which said bath comprises said phenol in solution in a substantially nonaqueous solvent. 1

20. The method recited in claim 19 in which said solvent is an alcoholic solvent.

, 21. The method recited in claim 16 in which said bath includes an alkaline electrolyte.

. 22.,The method recited in claim 21 in which said alkaline electrolyte is an alkali metal hydroxide electrolyte.

23. The method recited in claim 21 in which said alkaline electrolyte is an alkali metal salt of a weak acid as the electrolyte.

24. The method recited in claim 16 in which said phenol is selected from the group consisting of aromatic compounds of the benzene and naphthalene series not otherwise substituted but with hydrocarbon and halogen.

25. The.method recited in claim 16 in which said phenol is mono-nuclear and has a tertiary carbon atom linked to the aromatic ring para to the phenoliohydroxyl group of said phenol.

References Cited in the file of this patent UNITED STATES PATENTS 2,060,715 Arvin Nov. 10, 1936 2,135,368 Vagenius et a1 Nov. 1, 1938 2,496,933 Caldwell Feb. 7, 1950 2,613,230 Niederl Oct. 7, 1952 2,676,918 Pawlyk Apr. 27, 1954 2,680,713 Lindsey et al. June 8, 1954 2,726,204 Park et al. Dec. 6, 1955 OTHER REFERENCES 

16. A METHOD OF PRODUCING AN ADHERENT, INERT, INSOLUBLE AND HEAT-RESISTING CROSS-LINKED POLYMERIC SOLID FILM UPON A METAL SURFACE COMPRISING IMMERSING SAID METAL IN A SUBSTANTIALLY NON-AQUEOUS ELECTROLYTIC BATHCONTAINING AN AROMATIC PHENOL HAVING AT LEAST TWO UNSUBSTITUTED POSITIONS IN THE AROMATIC RING, SAID POSITIONS BEING SELECTED FROM THE GROUP CONSISTING OF POSITIONS ORTHO AND PARA TO THE PHENOLIC HYDROXYL GROUP OF SAID PHENOL, SAID PHENOL RETAINING ITS AROMATIC STRUCTURE WHEN PARTIALLY OXIDIZED AND SAID ELECTROLYTIC BATH COMPRISING PHENOLATE IONS OF SAID PHENOL, AND PASSING A UNIDIRECTIONAL ELECTRICAL CURRENT THROUGH SAID BATH AND THROUGH SAID METAL ANODE TO CAUSE SAID PHENOLATE IONS TO DISCHARGE AND SAID PHENOL TO POLYMERIZE, CROSS-LINK AND BE DEPOSITED AS A POLYMERIC FILM UPON SAID METAL 